Oscillation generator



SePt- 2, 1941 R. L. MILLER 2,254,852

osCILLATIoN GENERATOR Filed Jan. 5, 1959 Wvg/WOR @y HLM/LER PatenteclSept. 2, 1941 OSCILLATION GENERATOR Ralph L. Miller, Verona, N. J.,assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application January 5, 1939,Serial No. 249,419

(Cl. Z50-36) Claims.

The present invention relates to the production of electrical waves forsignaling or similar purposes, and more particularly to the productionoi low frequency waves.

A known type of oscillation generator is the socalled relaxationoscillator in which a condenser is alternately charged from a spacedischarge tube and discharged through a resistive circuit. This type ofgenerator is frequently used Where a non-sinusoidal wave is permissibleor desired such as in the generation of a wave that is rich in harmoniccontent. Where a single frequency wave is desired, oscillators whosefrequency is determined by a tuned circuit are commonly used.

In the production of waves of low frequency the tuned circuit type ofoscillator requires tuning or lter reactances of large magnitude, whilethe relaxation type of circuit as heretofore used produces impure ornon-sinusoidal waves.

Itis an object of the present invention to secure substantiallysinusoidal waves by means of an oscillation generator of the relaxationtype, that is, one using resistance and capacity to determine thefundamental period.

This object is achieved in accordance with the present invention in itspreferred form by use of two discharge tubes in tandem in the feedbackloop for securing the necessary phase reversals and interstage circuitsof substantially Zero phase shift at the fundamental frequency. Also theinterstage circuits have large shunt capacity which eifectivelysuppresses harmonics and results in the production of relatively puresinusoidal waves.

The invention will be more clearly understood from the followingdetailed description taken in conjunction with the drawing in which:

Figs. 1, 2 and 3 are schematic circuit diagrams of oscillation generatorcircuits according to the invention; and

Fig. 4 shows the phase-frequency characteristic of the interstagecircuits.

Referring rst to Fig. 1, the two space discharge devices I and 2 areconnected in the general manner of the Abraham and Bloch relaxationoscillator with interstage coupling condensers 3 and 4 and plate andgrid resistances 1, 8 and I2, I3. This circuit is, however, inaccordance with the invention, provided with large condensers 5 and 6connecting respectively the plates and the grids directly together. Theload IU may be coupled in any suitable manner and is indicated ascoupled by transformer 9 to the plate circuits. The plate voltage sourceis shown at I I.

Instead of using a single condenser 5 across the plates or a singlecondenser 6 across the grids it may be more convenient in some cases touse individual shunting condensers as shown in Fig. 2 at i5, IS, I'I andI8. The circuit of Fig. 2 also illustrates one use to which theoscillator has been put in practice. The problem was to supply a wavehaving a frequency of six cycles per second to the relaxation oscillatorof a synthetic speech producing system of the type shown in DudleyPatent 2,121,142, June 2l, 1938. The relaXation oscillator comprisingtube 20 in the right-hand portion of Fig. 2 is the relaxation oscillatorshown in Fig. 6 of the Dudley patent. The circuit, however, has beenmodified by the provision oi key 22 which when depressed opens its uppercontact and closes its lower Contact, thus serially including in thegrid circuit of the relaxation oscillator 20 the output terminals of thesix-cycle wave generator comprising tubes I and 2. This six-cycle waveso modifies the output wave at 2|, of the relaxation oscillator 2E! asto produce a vibrato effect adding naturalness to the voice in certaincases. This use of the oscillator of the invention as shown in Fig. 2 isgiven for illustrative purposes only since the invention is capable ofgeneral application.

The principle of operation of the circuit of the invention may be moreclearly understood from considering Figs. 3 and 4. Fig. 3 shows theoscillator portion of Fig. 2 redrawn to make the interstage circuitsmore apparent. It is seen that the plate circuit of tube I feeds intothe grid circuit of tube 2 through an interstage network which consistsof the series capacity 3 and a shunt arm on each side of this capacityconsisting in each case of a condenser in parallel with the resistance.The interstage network, therefore, is of pi configuration. Since thecircuit is entirely symmetrical the interstage network connecting theplate of tube 2 to the grid of tube I is of the same type anddimensions. It is understood that this would still be true if thecondensers I5 and I6 of Fig. 3 were combined into one condenser 5 ofFig. 1. Likewise the condensers I`I and I8 of Fig. 3 may be combinedinto the one condenser 6 of Fig. 1.

Fig. 4 shows the phase-frequency characteristic of these two interstagecircuits. Since the tubes I and 2 are in tandem in the feedback loop andsince each tube gives a phase shift of degrees, the two tubes togethergive the required 360 degrees or zero degree phase shift for producingoscillations. This places a requirement of zero phase shift for each ofthe two interstage networks. Referring to Fig. 4, it is seen that eachinterstage network has a phase shift varying at extreme frequenciesbetween plus 90 degrees and minus 90 degrees and this characteristicpasses rather sharply through the Zero phase shift point. This point hasbeen marked fo on the drawing to indicate that this is the frequency ofoscillation of the circuit. The fact that the phase shift characteristicpasses through the zero phase point rather steeply indicates that thecircuit operates stably at the frequency corresponding to Zero phaseshift. This is found to be the case in practice. v

Without the shunting condensers 5, of Fig. 1 or I5, I6, I1, i8 of theother figures the wave form produced would approach rectangular. If theshunting condenser or condensers were omitted from only the plate side,the resulting wave would be rounded on one side, lfor example on thefront let us say, but would still be angular on the opposite side.Ornission of the shunting condenser orvcondensers on the grid side onlywould result in a wave rounded on its opposite side, for example theback, but angular on the front side. Inclusion of the shuntingcondensers across both the plates and the grids results in theYproduction of a wave which is nearly sinusoidal in form. One reason forthis appears to be that the presence of these shunting condensersresults in the type of phase characteristic shown in Fig. 4 and thatthese large shunting capacities effectively shunt harmonic frequencies.In order for the circuit to have this type of phase characteristic theshunt capacity must have reactive impedance which at the frequency ofoscillation is smaller than or at least as small as the shuntresistance. In one case used by applicant for generating a six-cyclewave, RCA-56 tubes were used. Referring to Fig. 1, the resistances I2and I3 were each 125,000 ohms. Condensers 5 and E each had a capacity of.5 microfarad. Resistances l' and 8 were each 27,000 ohms, making atotal of 54,000 ohms of which 4,000 ohms were included between theoutput transformer taps, as illustrated in Fig. 2. Capacities 3 and Ilwere each .17 microfarad. In this case it will be seen that the reactiveimpedance of each of the condensers 5 and 6 at the oscillation frequencywas about 50,000 ohms, which is at least as small as or smaller than theresistances 'i and 8 or l2 and i3, respectively, paralleling them.

The invention is not to be construed as limited to the circuit detailsnor to the values that have been given but the scope is indicated in theclaims which follow.

What is claimed is:

Y l. An oscillation generator comprising two space discharge tubes eachhaving at least triode elements, crossed condensers from plates to gridsof opposite tubes, resistive impedance paths from each grid to eachcathode and from each plate to each cathode, said tubes, condensers andresistive impedance paths together forming an oscillation generatingsystem, means for altering the wave form of the generated wavecomprising a capacity connected between said grids which is largecompared to the capacity of either of said crossed condensers and a loadcircuit for the generated oscillations symmetrically coupled to saiddischarge devices.

2. An oscillation generator comprising two space discharge tubes eachhaving at least triode elements, crossed condensers from plates to gridsof opposite tubes, resistive impedance paths from each grid to eachcathode and from each plate to each cathode, said tubes, condensers andresistive impedance paths together forming an oscillation generatingsystem, means for altering the wave form of the generated wavecomprising a capacity connected between said plates which is largecompared to the capacity oi either of said crossed condensers and a loadcircuit for the generated oscillations symmetrically coupled to saiddischarge devices.

3. A relaxation oscillator comprising two space discharge tubes havingat least triode elements, crossed condensers from plates to grids ofopposite tubes, a common connection for the cathodes of said tubes, aresistance between each grid and cathode, a resistance between eachplate and cathode, and capacity connected from plate to plate whosereactive impedance at the frequency of oscillation is not greater thanthe order of magnitude of the sum of the resistance between each ,plateand cathode.

e. A relaxation oscillator comprising two space discharge tubes havingat least triode elements, crossed condensers from plates to grids ofopposite tubes, a common connection for the cathodes of said tubes, aresistance between each grid and cathode, a resistance between eachplate and cathode, and capacity connected from grid to grid whosereactive impedance at the frequency of oscillation is less than theorder of magnitude of the resistance measured from grid to grid.

5. A low frequency oscillation generator comprising two space dischargetriodes each having input terminals and output terminals, said triodesbeing connected with the anode of each coupled to the grid of the otherto form a closed path, and two interstage circuits for so connectingsaid triodes, each interstage circuit comprising only capacities andresistances including capacity in shunt across the input terminals andcapacity in shunt acrossv the output terminals of said triodes, saidshunt capacities being sufficiently large in relation to the otherelements of the interstage circuits to impart to the interstage circuitsa phase shift characteristic varying with frequency and passing throughthe value zero at the frequency of oscillation.

6. A two-stage oscillation generator, each stage having its outputcoupled to the input of the other stage, each stage including a spacedischarge device having an anode, a cathode and a grid, each devicehaving substantially degrees phase shift and two interstage circuits forcoupling each stage to the other, each interstage circuit having aseries branch extending from the plate of one device to the grid of thenext device and a shunt branch eXtending from apoint in the seriesbranch to a cathode of the respective device, each such interstagecircuit comprising capacity in its series branch and parallelresistance-capacity elements in its shunt branch, the magnitude of thecapacity in the shunt branch of each interstage circuit beingsuiiiciently large in relation to the other elements of the respectiveinterstage circuit to cause each such interstage circuit to have zerophase shift at the frequency of oscillation and to exhibit reversal ofphase in going through the frequency of oscillation.

7. A relaxation oscillator comprising two space discharge tubes havingat least triode elements, crossed condensers from plates to grids ofopposite tubes, a common connection for the cathodes of said tubes, aresistance between each grid and cathode, a resistance between eachplate and cathode, and a capacity connected from each plate to cathodeat least as large as the order of magnitude of the capacity of one ofsaid crossed condensers.

8. A relaxation oscillator comprising two space discharge tubes havingat least triode elements, crossed condensers from plates to grids ofopposite tubes, a common connection for the cathodes of said tubes, aresistance between each grid and cathode, a resistance between eachplate and cathode, and a capacity connected from each grid to cathode atleast as large as the order of magnitude of the capacity of one of saidcrossed condensers.

9. A relaxation oscillator comprising two space discharge tubes havingat least triode elements, crossed condensers from plates to grids ofopposite tubes, a common connection for the cathodes of said tubes,resistive impedance from each grid to cathode and from each plate tocathode, said tubes, condensers and resistive impedances constituting acircuit for generating Waves rich in harmonics, and means forsuppressing the harmonies and giving a substantially sine wave outputcomprising capacities in shunt relation to the grid-cathode andplate-cathode circuits of said tubes, said capacities being as large asthe order of magnitude of said crossed condensers.

10. A low frequency oscillation generator comprising two space dischargetriodes each having input and output terminals, said triodes connectedwith the anode of each coupled to the grid of the other to form a closedpath, and two interstage circuits for so connecting said triodes, eachinterstage circuit comprising only capacities and resistances andcomprising in particular series capacity between the anode of one triodeand the grid of the other and resistances in shunt across the outputterminals of said one device and across the input terminals of the othertogether with capacity shunted across each of said resistances, each ofmagnitude large enough in relation to the resistance which it shunts tocause the phase shift characteristic of the respective interstagecircuit to pass through the Zero phase at the frequency of oscillation.

RALPH L. MILLER.

